The present invention relates to a method for the preparation of a hydrophilic gel by monomer casting a hydrophilic monomer in a plastic mold employing a surface active agent in the monomer containing mixture employed.
Monomer casting of hydrophilic gels, i.e. of lightly cross-linked copolymers of hydrophilic monomers, has indisputable advantages in comparison to the manufacturing of shaped articles by mechanical processing or nonswollen gels and their subsequent swelling. When glass casting molds were used, it was possible to obtain castings with a perfect surface. However, as soon as casting models made of plastics began to be applied, particularly the molds made from polymeric olefins, e.g. polypropylene, microscopical surface defects often occurred which caused, for example, a weak gray turbidity of cast contact lenses. The microscopical investigation of castings and casting molds revealed that minute parts of gel were torn from the surface of the casting and adhered on the surface of mold. Most probably, grafting of the polymerizing gel on the polymer of the creating mold occurred. The surface of a mold made from a crystalline hydrocarbon was affected by grafting above all in the places of amorphous areas of the olefin polymer, which formed palisade structures on the mold surface and a replica of which was distinguished microscopically on the torn-out parts of the gel. To prevent these defects, it has been recommended to carry out the polymerization at a higher temperature, e.g. at 110.degree.-135.degree. C. Though this defect was removed in this way, another problem occurred because a spontaneous deformation of the molds took place at a temperature above the second-order transition and near the melting temperature of the olefin polymer. This caused serious defects, above all in the optics of cast contact lenses.